Field of the Invention
The present invention relates to a condenser microphone unit that can be used for, e.g., a narrow directional microphone equipped with an acoustic tube, and to a method of manufacturing the condenser microphone unit.
Description of the Related Art
FIG. 7 is a cross sectional view of a typical unidirectional condenser microphone unit. The condenser microphone unit illustrated in FIG. 7 includes a unit case 10 including a plurality of front acoustic terminal holes 10a on the front end surface thereof, an electroacoustic transducer 20 contained in the unit case 10, and a circuit board 30 disposed on the rear end opening of the unit case 10.
The electroacoustic transducer 20 includes a diaphragm 22 stretched across a supporting ring 21 with a predetermined tension, a disk shaped fixed electrode 24 supported on a face side of an insulation base 25, and a spacer ring 23 having electrical insulating property disposed between the diaphragm 22 and the fixed electrode 24.
As illustrated in the drawing, the diaphragm 22 and the fixed electrode 24 are disposed to oppose each other with the spacer ring 23 therebetween to form an electrostatic electroacoustic transducer 20.
A field-effect transistor (FET) 40 serving as an impedance converter is mounted on the circuit board 30.
The circuit board 30 includes a rear acoustic terminal 32. Acoustic holes (acoustic wave introducing holes) 25a and 24a are drilled in the insulation base 25 and the fixed electrode 24, respectively.
This configuration allows acoustic waves traveling from the rear acoustic terminal 32 to have effect on the back side of the diaphragm 22 via the acoustic holes 25a and 24a. 
A predetermined acoustic resistance material 26 is disposed in the air chamber 50 provided between the fixed electrode 24 and the acoustic hole 25a. 
By connecting an acoustic tube (not shown) to the front face side of the microphone unit, the microphone unit can be used as a microphone having narrow directional property.
However, the condenser microphone equipped with an acoustic tube having narrow directional property has disadvantage that narrow directional property cannot be provided by using the acoustic tube at low frequency because of the dimensional relationship between the length of the acoustic tube and the wavelength of acoustic waves. So that, for low frequencies where the acoustic tube does not work, an acoustic tube is connected to a front acoustic terminal of a unidirectional unit to operate the microphone as a unidirectional microphone. A microphone having narrow directional property equipped with an acoustic tube is disclosed in JP 2000-050386 A.
In the narrow directional microphone as described above, the effective distance between acoustic terminals at low frequency band is long, so that the acoustic mass of the acoustic tube is connected to the front side of the diaphragm 22 of the unidirectional condenser microphone unit. Thus the directional property of the unit should be adjusted to have directional frequency response almost identical to omnidirectional property when measured in a free space.
The air chamber 50 in the rear side of the fixed electrode 24 drives omnidirectional elements to the diaphragm 22 and determines the equivalent mechanical mass of the diaphragm 22 and a resonance frequency of the stiffness of the air chamber 50. To achieve the design providing a resonance frequency at a high limit of a sound collection band, the air chamber 50 should be designed to have a small volume to increase its stiffness.
In addition, to obtain directional frequency response almost identical to omnidirectional property, the acoustic resistance of the rear acoustic terminal 32 should be increased to reduce the force that drives bidirectional elements to the rear side of the diaphragm 22 from the rear acoustic terminal 32. Since the air chamber 50 has high stiffness, the acoustic resistance of the rear acoustic terminal 32 is designed to be very high.
However, when leakage occurs between the rear side of the diaphragm 22 and the acoustic resistance of the rear acoustic terminal 32, the effective acoustic resistance during operation is reduced and a problem arises that the intended directional property cannot be achieved.
Specifically, as illustrated in FIG. 8, the problem is the leakage from the contact portion (contact portion at the rim portion) between the insulation base 25 and the fixed electrode 24.
The fixed electrode 24 is usually punched out from a metal plate having an electret material (FEP film) thermally bonded thereto, so that the fixed electrode 24 has a rough end surface with a sheared surface 61 and a sharp edge 62.
The insulation base 25 is usually manufactured by injection molding of polycarbonate (PC). For the insulation base 25, shrinking of material during cooling produces roughness on the surface which is to make contact with the fixed electrode 24. These rough surfaces produced during manufacturing disadvantageously create a leak passage 63 between parts.
Moreover, variation in dimensions of the leak passage 63 disadvantageously causes difference in directional frequency response at low range among manufactured microphones. In particular, for a condenser microphone equipped with a long acoustic tube having narrow directional property, the leakage causes disadvantageous effects and has become a serious problem.